The molecular rotation parameters of cholesteric liquid crystals (LCs) are calculated based on their phase transition temperatures using a semi-empirical molecular orbital method. A molecular rotation model is employed to evaluate the correlation between the LC phase transition and their molecular structure. The cholesteric-isotropic phase transition temperature and critical rotational velocity are regularly reduced with increasing molecular length. A novel explanation for the molecular interaction effect on phase transition is proposed. For low-temperature phases, the molecular interaction is strong and plays a key role in the phase transition. In contrast, for high-temperature phases, the molecular rotation effect becomes the important factor for the phase transition.